The cop sounded reasonably sincere about your safety although he was not entirely correct since there are no minimum speed limits on most roads other than highways. Still, if I could I'd stay away from the traffic.

I'd stay out of the traffic lane as much as reasonably possible. I have a mirror and if I see a large vehicle or a buildup of traffic behind me I pull over to the right if it's safe and wave them to pass me. I figure it doesn't cost me that much and people might appreciate that and maybe cut some cyclist a slack later. I don't believe in self-righteous exercising of my right at any cost without regards for my own safety and convenience of other people. If the speed limit is 40mph and the traffic goes at that speed, you will block the traffic, no doubt, and even though you might be within legal limits to ride at 15mph, I'd try to get out of their way. I know the cop wouldn't tell a tractor going at 15mph to get off the road, so they shouldn't say the same to a cyclist. But, unlike the tractor, we do have the ability to move aside from time to time and let the traffic pass.

Like most things in life riding a bike in traffic is about balance and compromise. You need use your judgment when to exercise you rights and when to get out of the way.

Adam